David Burton writes:
I just realized the obvious answer to a question that has been nagging in the back of my mind for nearly a year and a half.
In 2008 Margaret Zimmerman asked two questions of 10,257 Earth Scientists at academic and government institutions. 3146 of them responded. That survey was the original basis for the famous “97% consensus” claim.
For the calculation of the degree of consensus among experts in the Doran/Zimmerman article, all but 79 of the respondents were excluded. They wrote:
“In our survey, the most specialized and knowledgeable respondents (with regard to climate change) are those who listed climate science as their area of expertise and who also have published more than 50% of their recent peer-reviewed papers on the subject of climate change (79 individuals in total). Of these specialists, 96.2% (76 of 79) answered “risen” to question 1 and 97.4% (75 of 77) answered yes to question 2.”
The basis for the “97% consensus” claim is this excerpt:
[of] “the most specialized and knowledgeable respondents (with regard to climate change)… 97.4% (75 of 77) answered yes to question 2.”
But that is a false statement.
The two questions were:
Q1: “When compared with pre-1800s levels, do you think that mean global temperatures have generally risen, fallen, or remained relatively constant?” 76 of 79 (96.2%) answered “risen.”
Q2: “Do you think human activity is a significant contributing factor in changing mean global temperatures?” 75 of 77 (97.4%) answered “yes.”
My nagging question was, why did different numbers of people (79 vs. 77) answer the two questions? What happened to the other two respondents?
The answer to that question is not in the Doran article.
But it is in the Zimmerman report, a copy of which I bought back in March, 2012. The reason I feel stupid is that I read it and even quoted the relevant part way back then, and it still took me until now to realize the obvious answer to my nagging question.
This was the full set of questions that Zimmerman asked in their survey:
Q1. When compared with pre-1800's levels, do you think that mean global temperatures have generally risen, fallen, or remained relatively constant?1. Risen2. Fallen3. Remained relatively constant4. No opinion/Don't knowQ2. Do you think human activity is a significant contributing factor in changing mean global temperatures? [This question wasn't asked if they answered "remained relatively constant" to Q1]1. Yes2. No3. I'm not sureQ3. What do you consider to be the most compelling argument that supports your previous answer (or, for those who were unsure, why were they unsure)? [This question wasn't asked if they answered "remained relatively constant" to Q1]Q4. Please estimate the percentage of your fellow geoscientists who think human activity is a contributing factor to global climate change.Q5. Which percentage of your papers published in peer-reviewed journals in the last 5 years have been on the subject of climate change?Q6. AgeQ7. GenderQ8. What is the highest level of education you have attained?Q9. Which category best describes your area of expertise?
Do you see it? If a respondent answered “remained relatively constant” to the first question, then he wasn’t asked the second question!
That’s obviously why only 77 answers were reported to the second question. Two of their 79 top climate specialists had answered “remained relatively constant” to the first question, and those two were not asked the second question, and were not included in the calculation of the supposed 97.4% agreement.
That means only 75 of 79 (94.9%) of their “most specialized and knowledgeable respondents” actually gave them the answers they wanted to both of their questions.
So, despite asking “dumb questions” that even most skeptics would answer “correctly,” and despite excluding over 97% of the responses after they were received, they still did not find 97% agreement. They actually found only 94.9% agreement.
I’ve updated my http://tinyurl.com/Clim97pct page to reflect that fact.
I’ve also emailed the editor of Eos, which published their article back in 2009, asking that they run a correction.